Masts and towers are widely used to overcome the short ground level line of sight range of devices which exploit the electromagnetic spectrum, including radio systems, electro-optical systems, TV transmitter and receiver antennas, and many more. The restriction on range occurs due to the effect of the curvature of the earth, and, in many cases, due to local obstructions. Usually, cables are necessary to run the electromagnetic and/or optical signals from the device at the top of the mast to ground level.
In addition to static masts at fixed locations, such as cell towers, there exist many needs for transportable, quick set up masts and towers for temporary, emergency, or military use. Some of these—mostly vehicle-mounted masts of telescopic design—are intended for “push-button” operation, where the operator's only task is to press a button to raise or lower the mast.
To provide a mobile push-button system which requires no additional intervention by the user, an automatic cable management system is necessary. De facto, the cable(s) must remain connected at upper and lower ends. This requirement usually rules out the use of cable reels (which would necessitate disconnection of the lower end of the cables as the mast extends and retracts). An effective cable management method is to form the cable into a helical coil wound around the mast. If the coil is sufficiently long, both lower and upper ends may remain connected at all times, and the mast may be extended to full height with the helix simply increasing in length while reducing in diameter in compensation.
For decades, since the early days of Outside Broadcast and Electronic News Gathering, it has been common industry practice to use coiled air hose as a housing conduit for the cable systems used in conjunction with pneumatic telescopic masts installed on mobile communications vans. Typical air hose is made from polyurethane, polyethylene, or nylon, as is known in the art. Air hose is used in a wide range of industrial applications for the transfer of air, water, or moderate chemical solutions. Coiled air hose is intended for use for the same industrial applications, but, because it resists kinking, and occupies minimum space when coiled up, it has been used for other applications when those two features are of value. These early cable management methods were used with military mobile communications and intelligence systems around the same time. The principle reason for the adoption of this technique is that the air hose, which operates much like a spring, has a memory, exhibiting a preference for the helical shape. As a result, the assembly assists in keeping the cables where they need to be in relation to the mast, whether in the retracted state, during extension/retraction, or when fully extended.
A problem with industrial air hose is that it is not intended to be used as a conduit for cables, and, in some cases, the manufacturer is unwilling to support the use of its product for this application. Thus, there is no warranty, and often, no detailed technical information regarding the mechanical performance under extreme environmental conditions.
Second, the largest industrial air hose available has an internal diameter of only 1¼ inches. This diameter severely limits the number and size of cables that can be housed in the conduit. Air hose manufacturers have expressed little interest, if any, in manufacturing larger hoses for housing conduit use. Mast system designs are thus limited in terms of cabling.
Third, the coiled air hose minimizes the space occupied when not in use, and therefore comes in a tight coil that exhibits spring-like behavior. A certain amount of force is required to extend the coiled air house. This spring force directly opposes the motive force of the mast extension mechanism. Put another way, the effect of the spring force is to increase the payload weight of the mast. One result is that the telescoping mechanism must be designed with sufficient strength to extend the mast at significantly larger loads than those imposed solely by the payload, thereby increasing the overall size and cost of the assembly.
Another drawback is that installation of cables within an air hose is a difficult and time-consuming procedure, provided by a limited number of specialist practitioners. At least one air hose manufacturer claims the installation procedure weakens the air hose.
A further drawback is that, once installed, individual cables are not easily replaced in the event of a continuity problem. In many cases, it is more cost effective (but none-the-less expensive) to replace the entire assembly, including air hose.
The most dramatic disadvantages of using an industrial air hose for cable management arise in outdoor applications at low temperatures. The mechanical properties of most air hoses vary significantly with temperature. In temperate climate zones, this temperature dependency becomes apparent in wintertime. For example, nylon and polyurethane air tubes exhibit a significant increase in spring force as temperature decreases. This phenomenon increases the effective weight of the mast payload. In some cases, such as at the low end temperatures commonly specified for military applications (e.g., about −40° C./F.), the spring loads imposed by the cable assembly conduit alone may be in the hundreds of pounds. Because of the temperature-dependent properties of air hoses, the effective payload weight can increase at low temperatures, sometimes to the point where the capabilities of the mast and/or its drive mechanism are exceeded.
Another adverse effect of increasing spring force with reduced temperature is the twist force imposed by the helical air hose as the mast extends. This twist force is a function of the spring force consequent upon the extension of any coil whose ends are secured, so large forces are highly undesirable. In the case of a telescoping mast, the lowest tube section is secured by the mast mounting arrangements, so any twist forces tend to twist the top of the mast (which is cantilevered). This twisting tends to create difficulties where accurate payload pointing angles are required (such as narrow beam antennas, or narrow field of view optical systems). In extreme conditions, such twisting forces may result in mechanical/structural problems.
Typical air hoses used as cable conduits become increasingly rigid as temperatures decreases. If exposed to cold sink conditions, they tend to exhibit great reluctance to retract neatly, and may require manual intervention to properly stow as the mast column retracts. Such manual intervention will often require a crew member to climb up on the vehicle, sometimes to a precarious position. This clearly is a major drawback in a system designed for pushbutton operation.
In addition, at cold temperatures, the air hose tends to become brittle. There are many reports from the Electronic News Gathering community regarding embrittlement and consequential fracturing of the hose during or after exposure to cold weather, usually at the upper and lower extremities of the assembly. Conduit fracturing tends to happen at these locations because, as the working length of the coil is shortened or lengthened as the mast moves up and down, the conduit flexes as it exits rigid attachment brackets, inducing fatigue at those points.
What is needed is an alternative to housing conduit in telescoping masts that overcomes the many shortfalls of the common air hose used in conventional systems. In particular, a cable management assembly with a cable assembly restraint device is needed that maintains the assembly in the desired shape, such as a helical coil in telescoping mast applications, that is designed for such applications, reducing the cost, size, and load of the overall assembly, and meets performance requirements throughout the range of potential temperature conditions.